It is well known in the art that the resistance modulation of Hall elements or magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies.
Accurate engine crank position information is needed for ignition timing and OBDII mandated misfire detection. Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,714,883, 5,731,702, and 5,754,042).
The crank position information is encoded on a rotating target wheel in the form of teeth and slots. The edges of the teeth define predetermined crank positions. The sensor is required to detect these edges accurately and repeatably over a range of air gaps and temperatures. Virtually all such sensors are of the magnetic type, either variable reluctance or galvanomagnetic (e.g. Hall generators or magnetoresistors). Galvanomagnetic sensors are becoming progressively most preferred due to their capability of operation down to a zero speed, greater encoding flexibility and standardized output signals.
Furthermore, temperature and the size of the air gap affect the output signal of a magnetic sensing element. Consequently, operation over wide temperature and air gap size ranges requires some form of compensation for the resultant signal drift, both in amplitude and offset. The most common approach is the use of two matched sensing elements operating in a differential mode thereby providing a common mode rejection.
An example of such a sensor is the sequential crankshaft sensor used on several of General Motors Corporation trucks. This sensor employs two matched InSb magnetoresistor elements located radially proximate to the target wheel, one being slightly displaced with respect to the other in the direction of target wheel rotation. FIG. 1 is a schematic representation of an exemplar automotive environment of use according to this prior art scheme, wherein a target wheel 10 is rotating, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative position thereof is to be sensed. Rotative position of the target wheel 10 is determined by sensing the passage of a tooth edge 12, either a rising tooth edge 12a or a falling tooth edge 12b, using a single dual MR differential sequential sensor 14. A tooth edge 12 is considered rising or falling depending upon the direction of rotation of the target wheel 10 with respect to the magnetoresistive sensors MR1 and MR2. MR1 is considered leading and MR2 is considered lagging if the target wheel 10 is rotating in a clockwise (CW) direction whereas if the target wheel is rotating in a counterclockwise (CCW) direction then MR1 is considered lagging whereas MR2 is considered leading. For purposes of example, the target wheel 10 will be assumed to be rotating in a CW direction in the views.
The single dual MR differential sequential sensor 14 employs two matched magnetoresistor elements, MR1 and MR2, which are biased by a permanent magnet 16, wherein the magnetic flux 18 and 20 emanating therefrom is represented by the dashed arrows. The magnetic flux 18 and 20 passes from the permanent magnet 16 through the magnetoresistors MR1 and MR2 and through the air gaps 22 and 24 to the target wheel 10. The target wheel 10 is made of a magnetic material having teeth 26 and spacings 28 therebetween. The spacing L between MR1 and MR2 is generally such that the trigger points for the rising and falling edges of the output signal V.sub.OUT as shown in FIG. 2C are dependent on the leading MR only as depicted in FIGS. 2A, 2B and 2C, as will be later described.
Power is supplied to CURRENT SOURCE130 and CURRENT SOURCE232 through voltage source 34. Power is also supplied to a comparator 36 (with hysteresis) through voltage source 34 but is not shown. CURRENT SOURCE130 supplies current to MR1 thereby providing for an output voltage V.sub.MR1 from MR1. CURRENT SOURCE232 supplies current to MR2 thereby providing for an output voltage V.sub.MR1 from MR2. Output voltages V.sub.MR1 and V.sub.MR2 are input into the comparator 36 whose output voltage V.sub.OUT, as shown in FIG. 2C, is an indication of the position of rotation of the target wheel 10. It is to be understood that all voltages are measured with respect to ground unless otherwise indicated herein, and that CURRENT SOURCE1 is matched to CURRENT SOURCE2.
As shown in FIG. 2A, the lagging MR element, in this case MR2, provides a delayed signal in every respect identical to the signal from the leading MR, in this case MR1. The differential signal V.sub.D =V.sub.MR1 -V.sub.MR2, shown in FIG. 2B is electronically generated within the comparator 36 and is then used by the comparator to reconstruct the signal V.sub.OUT (shown in FIG. 2C) emulating the profile of the target wheel 10. Upon a closer inspection of FIGS. 2A, 2B and 2C, it becomes evident that the rising edges 42 and the falling edges 44 of the sensor output signal V.sub.OUT are determined only by first points 46 corresponding to the rising edges and second points 48 corresponding to the falling edges where the signal from the leading MR, in this example MR1, crosses a first threshold voltage 50 corresponding to the first points and a second threshold voltage 52 corresponding to the second points wherein the first and second threshold voltages are determined by the hysteresis applied to the comparator 36. The lagging MR, in this example MR2, has no part in the generation of the rising edges 42 or the falling edges 44 of the output signal V.sub.OUT. The lagging MR simply determines the offset voltage 54 of the leading MR.
What is needed is a method and apparatus wherein a single element sensor, preferably, but not exclusively, a single element magnetoresistive sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel.